High-resolution x-ray diffraction patterns of crystalline transfer RNA that show helical regions

Yeast phenylalanyl transfer RNA crystallizes in a simple orthorhombic unit cell (a = 33.2, b = 56.1, c = 161 A), and the crystal yields an x-ray diffraction pattern with a resolution of 2.3 A. From an analysis of the packing in the unit cell it is concluded that the molecular dimensions are approximately 80 by 33 by 28 A. The diffraction pattern viewed along the a-axis has a distribution characteristic of double-helical nucleic acids. However, this distribution is not found when the pattern is viewed along the b-axis. This has been interpreted as indicating that the double-helical portions of the transfer RNA molecule are approximately half a helical turn in length, and therefore can contain 4-7 base pairs. These results are consistent with the cloverleaf formulation of transfer RNA secondary structure.